Ace the 2026 IAC Science Bee – Shine Bright with the White Set Challenge!

This question is about how periods on the periodic table relate to electron shells. The number of a period matches the principal energy level that is being filled in the ground-state electron configurations. Lithium through neon all have their outer electrons in the second shell (n = 2). For lithium, the configuration is 1s2 2s1, and for neon it’s 1s2 2s2 2p6. Since their outermost electrons occupy the second shell, they belong in the second period. The next elements start filling the third shell (period three), the following set fills the fourth shell, and so on.